I am remodeling a bath that has wiring for an existing switch and old light (which has an electric plug attached). I want to put in a new switch to control the new light and additionally install a GFI outlet. How do I convert my existing switch to light wiring to a switch that turns on/off light, but does not control GFI.

Shut the power off and pull the switch and light out. One box will have a single cable, and the other, 2 or more. One of the cables in the box with 2 or more is the feed wire. You need identify which one and connect the cable to the GFI outlet line terminals to it along with what ever it was connected to before. The outlet will have power with the switch on or off.